CRONTAB, is a time-based job scheduler in Unix like operating systems. 
In laymen term, it is used to execute scripts on your machine at a predefined time or interval. 
This can be any script, for instance downloading images from a particular website every hour or generating a pdf of your unread emails from the past day and the list goes on


###################################
BASICS
###################################

Each line of a crontab represents a job and it looks as follows:

* * * * * command_name

Each asterisk here has a special meaning :
Asterisk 1 : Represents minutes from 0 to 59
Asterisk 2 : Represents hour of the day from 0 to 23
Asterisk 3: Represents day of the month from 1 to 31
Asterisk 4: Represents a month from 1 to 12
Asterisk 5: Represents the day of the week from 0 to 6 (Sunday to Saturday)
Command : A command that executes a script

Example : 0 10 * * * /bin/bash print_time.sh
You can play with these settings from the following site : https://crontab.guru

The above example will execute the script print_time.sh using the /bin/bash shell, everyday at 10:00 A.M. 
Crontab uses a 24 hour clock
Providing the full path to the script is necessary
Providing a shell name or a specific interpreter is necessary too

We can also use the following strings to define job.

@reboot	Run once, at startup.
@yearly	Run once a year.
@annually	(same as @yearly).
@monthly	Run once a month.
@weekly	Run once a week.
@daily	Run once a day.
@midnight	(same as @daily).
@hourly	Run once an hour.

@reboot <command-to-execute> # run a job every time the system reboots

###################################
CREATING CRONTAB JOBS
###################################

crontab -e # this opens up the crontab file in your predefined editor

# create a basic cronjob that runs everyday at 10:00 A.M.
0 10 * * * /bin/bash ~/Documents/collect_log.sh 

- this cronjob will get executed everyday at 10:00 AM
- note the leading 0 is important
- /bin/bash is the interpreter here, and this is required
- ~/Documents/collect_log.sh is the absolute path to the script to be executed



###################################
VIEW CRONJOBS FOR THE CURRENT USER
###################################
crontab -l 


##########################################
REMOVE ALL CRONJOBS FOR THE CURRENT USER
##########################################
crontab -r